The invention relates to the use of focused ion beams (FIB) in the preparation of samples comprising discrete regions of heterogeneous materials for viewing with electron microscopy and, more particularly, to the preparation of sections of magnetic heads for viewing with a scanning electron microscope (SEM).
A typical prior art head and disk system is illustrated in FIG. 1. In operation the head 10 is supported by a suspension 13 as it flies above the disk 16. The magnetic transducer 10, usually called a xe2x80x9chead,xe2x80x9d is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths 14 which are attached to or embedded in the suspension arm (not shown). Typically there are two electrical contact pads each (not shown) for the read and write heads 12, 23. Wires or leads (not shown) are connected to these pads and routed in the suspension 13 to the arm electronics (not shown). The disk 16 is attached to the spindle 22 that is driven by spindle motor 24 to rotate the disk. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films include ferromagnetic material that is used to record the magnetic transitions in which information is encoded.
The write head 23 portion of the transducer 10 is further illustrated in FIG. 2. FIG. 2 is a section view of the write head 23 taken parallel to the air bearing surface which is not shown. The write head 23 includes two pole pieces which are referred to as P131 and P232 and a coil (not shown). To decrease the side writing and, therefore, to reduce the track width, the pole pieces 31, 32 are shaped into narrow tips at the gap 33. To be effective the tip of P131 should be very close to the same size as the tip of P232 and should extend up from the larger body of P131 pole piece about 1 to 1.5 times the gap 33 thickness. In one prior art method P131 is deposited first and initially has a broad, flat tip that is subsequently ion milled using the tip of P232 as a mask to form the tip of P131. U.S. Pat. No. 6,111,724 to Hugo Santini discusses a prior art process for making P232 tips and describes an improvement using a zeroxe2x80x94throatxe2x80x94height defining layer.
Regardless of the method used to form P232, the width of the track written by this type of inductive head 23 is largely determined by the width of the bottom of P232 (P2b). P232 tends to be wider at the top (away from the gap 33) which creates an additional complication in measuring the width of P2b. It is important to be able to measure P2b with some precision to monitor the manufacturing process. There are numerous variables in the process which affect the formation and shape of P232 including those affecting the photolithography used to define the shapes, the plating process used for depositing the ferromagnetic material, the seed layer removal process and the ion milling used to shape P131 using P232 as a mask. These variables can change from time to time in the manufacturing process and may even vary across a single wafer (not shown).
One prior art method used to measure the width of P2b uses a FIB to cut a section in the write head 23 to expose the tip of P232, the gap 33 and the tip of P131. In FIG. 2, a thin film layer of protective material 37 such as tungsten (W) or platinum (Pt) is deposited to preserve the P232 outline while a hole (not shown) is being cut. The hole is cut with a perpendicular incidence to expose the section illustrated in FIG. 2 as a substantially planar sidewall of the hole. An SEM beam is then used at an angle off of perpendicular to image the sidewall. The SEM image thus obtained will contain an image of the P232 and P131 tips. However, this prior art method is deficient in that it fails to yield truly planar side walls since the different materials that make up the sample of the pole piece tips have significantly different FIB etch rates. For example, the head fabrication process typically creates a thin redep layer (not shown) on the sides of P232 which has a higher etch rate than the NiFe which is commonly used for pole piece tips and the tungsten protective material has a lower etch rate than the NiFe. Voids in the tungsten can also contribute to variations in etch rate. Since the FIB has a very small diameter and is rastered, the higher etch rate materials will be cut deeper and the surface will not be planar. This topography can contribute to undesirable contrast. In the samples of pole piece tips, the top of the P232 is several microns higher than the surrounding field region. Since the beam begins to cut on the field at the same time as the top of P2, the field region will be greatly recessed relative to the P232. Therefore, the ending surface will have nonplanarity which follows the P2 pattern. The nonplanarity of the cut surface obscures contrast in the SEM image at the boundary between the different materials (for example, NiFe and the W) that define the critical dimension to be measured. The nonplanarity introduces undesirable contrast that makes it difficult to measure contrast due to the boundaries between the NiFe P232 and the tungsten coating 37 in the SEM image. Since the width of the NiFe P232 is a critical dimension, it is important to be able to measure it precisely.
Thus, there is a need for an improved process for preparing the pole piece tip samples for imaging.
The improved process according to the invention prepares samples for imaging by directing the FIB beam so that its incident angle is not parallel to the planar boundaries between various materials including boundary between the protective material deposited over the structure and outside material of the structure. This has the effect of evening out the etch rate, since the majority of the key beamlets cut more than one type of material. The resulting sample surface is easier to interpret than one produced by the prior art since the obscuring effect of curtaining is reduced. This allows greater accuracy of measurement from the image obtained by an SEM. A method according to a preferred embodiment of the invention is used to prepare a P2 tip so that the bottom width may be more accurately measured. A sample magnetic transducer with the P2 tip exposed is prepared for imaging by first depositing a protective material such as tungsten or platinum over the P2 tip (which is a ferromagnetic material such as NiFe). The sides of P2 tip and the protective material have contact planes which are perpendicular to the general plane of the upper surface of the sample. The sample is positioned in relation to the focused-ion beam so that most of the key beamlets cut at least two materials, for example, tungsten and NiFe, in the planar contact region to reduce the curtaining effect caused by the unequal rates of etching. The focused-ion beam is then used to etch away material to expose a new surface on which the cross section of the P2 tip is exposed for subsequent imaging.